Practical Measurements of Mechanical Properties

The first group of tests is carried out on specimens generally fabricated into a dumb-bell shape, with forces applied uniaxially. The usual apparatus consists of a machine with a pair of jaws, which during the test are moved relative to each other, either together or apart, in a controlled manner. A chart recorder is employed to give a permanent record of the results obtained, so that the force at fracture can be determined. Whether this kind of set up measures tensile, compressive, or flexural strength depends on how the sample is oriented between the jaws, and on the direction that the jaws are set to travel relative to one another. 

All of these tests, by their nature, need to be repeated several times with different specimens of any polymer sample, in order to ensure that there is enough information for statistical analysis. Although the physicist Lord Rutherford said that if your results need statistics, you ought to have done a better experiment, his dictum cannot be extended to tests on mechanical 

Dynamic mechanical tests, which are the other major group of testing techniques, tend not to be subject to such wide variation in results as nondynamic ones, and hence statistics tend not to be used to the same extent. 

This second group of tests is designed to measure the mechanical response of a substance to applied vibrational loads or strains. Both temperature and frequency can be varied, and thus contribute to the information that these tests can provide. There are a number of such tests, of which the major ones are probably the torsion pendulum and dynamic mechanical thermal analysis (DMTA). The underlying principles of these dynamic tests have been covered earlier. Such tests are used as relatively rapid methods of characterisation and evaluation of viscoelastic polymers, including the measurement of T, the study of the curing characteristics of thermosets, and the study of polymer blends and their compatibility. They can be used in essentially non-destructive modes and, unlike the majority of measurements made in non-dynamic tests, they yield data on continuous properties of polymeric materials, rather than discontinuous ones, as are any of the types of strength which are measured routinely. 

Overall, in order to characterise a polymeric material completely, data from both kinds of test are needed. 

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One of the more recently exploited forms of thermal analysis is the group of techniques known as thermomechanical analysis (TMA). These techniques are based on the measurement of mechanical properties such as expansion, contraction, extension or penetration of materials as a function of temperature. TMA curves obtained in this way are characteristic of the sample. The technique has obvious practical value in the study and assessment of the mechanical properties of materials. Measurements over the temperature range - 100°C to 1000°C may be made. Figure 11.19 shows a study of a polymeric material based upon linear expansion measurements. 

For the materials scientist the measurement of mechanical properties provides information about internal structure and such measurements afford an opportunity to test theories of structure. In consequence a range of tests has been developed by materials scientists which supplement the, usually simpler, engineering tests and which in many cases provide information of no obvious value to the engineer. There are, however, many ad hoc tests used by the practical man which provide information, applicable by the materials scientist, but often in a confusing or complicated way. 

Tighe has contributed a chapter on materials to the recently published text on contact-lens practice edited by Stone and Phillips. The measurement of mechanical properties of materials in this field has been discussed by the same authorand others. Although an interdependence of individual tear-fluid chemistry, surface... 

The most important displacive transformation is the one that happens in carbon steels. If you take a piece of 0.8% carbon steel "off the shelf" and measure its mechanical properties you will find, roughly, the values of hardness, tensile strength and ductility given in Table 8.1. But if you test a piece that has been heated to red heat and then quenched into cold water, you will find a dramatic increase in hardness (4 times or more), and a big decrease in ductility (it is practically zero) (Table 8.1). 

Much of the experimental work in chemistry deals with predicting or inferring properties of objects from measurements that are only indirectly related to the properties. For example, spectroscopic methods do not provide a measure of molecular stmcture directly, but, rather, indirecdy as a result of the effect of the relative location of atoms on the electronic environment in the molecule. That is, stmctural information is inferred from frequency shifts, band intensities, and fine stmcture. Many other types of properties are also studied by this indirect observation, eg, reactivity, elasticity, and permeabiHty, for which a priori theoretical models are unknown, imperfect, or too compHcated for practical use. Also, it is often desirable to predict a property even though that property is actually measurable. Examples are predicting the performance of a mechanical part by means of nondestmctive testing (qv) methods and predicting the biological activity of a pharmaceutical before it is synthesized. 

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